1. Field of the Invention
This invention relates to a liquid crystal electro-optic device such as a liquid crystal display device, a liquid crystal optical shutter array and the like, and more particularly to a liquid crystal electro-optic device whose display and driving characteristics are improved using an effective orientation of liquid crystal molecule.
2. Prior Art
As a conventional liquid crystal electro-optic device has been known a device using twisted nematic liquid crystal (hereinafter referred to as "TN liquid crystal"). The TN liquid crystal has a weak point that it generates a crosstalk in a matrix-type driving control using high pixel density. In addition, an active matrix display device which has a thin film transistor assigned to each pixel is also well known, but it still has problems that it is more difficult to obtain high production yield as a device size becomes larger, and that it also requires an enormous plant investment.
On the other hand, ferroelectric liquid crystal which was found by Myer et al. in 1977 is attracting public attention and studied intensively because of specific properties thereof.
FIG. 3 shows an orientation state of ferroelectric liquid crystal which has been generally accepted. Ferroelectric liquid crystal material interposed between a pair of substrates has a layered structure, and liquid crystal molecules thereof are spirally oriented along a side surface of each corn. For use of such a ferroelectric liquid crystal in a liquid crystal electro-optic device such as a display device, there have been proposed various modes such as a surface-stabilizing mode disclosed by Clark or Lagerwell, a scattering mode disclosed by Yoshino, or a mode using antiferroelectric liquid crystal, etc. All modes as described above utilize a common feature of the ferroelectric liquid crystal which would develop when the spiral structure of ferroelectric liquid crystal molecules is broken by some methods.
One of these methods is to narrow a gap of the substrates or applying strong electric field to the ferroelectric liquid crystal. Particularly, a method disclosed in U.S. Pat. No. 4,367,924 by Clark has a condition that a gap interval between the substrates is required to be shorter than a distance at which the spiral structure is formed and to be five times or less of a spiral pitch. The gap interval satisfying the above condition enables a liquid crystal state to become a surface-stabilized ferroelectric liquid crystal state, and thus the ferroelectric liquid crystal device develops attractive features thereof such as a storing property (memory property), high response speed and so on. However, this method positively utilizes a property of the substrates of suppressing the formation of a spiral structure of the ferroelectric liquid crystal, and therefore various characteristics of the device are liable to be affected by the substrates or an orientation control means provided on the substrates. For example, switching characteristic of the device is affected by them so that there frequently occurs that the motion of the liquid crystal molecules is not uniform when the liquid crystal molecules are driven by a plus electric signal and a minus electric signal having the inverted waveforms to each other (symmetric electric signals).
In addition, these characteristics of the liquid crystal are not invariable ones at all times, but are timevariable ones, so that the device whose switching operation is stabilizingly carried out has not been obtained yet.